IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 124 NOM and Aquatic Systems Thursday, 27 August 2026 / Hall C SL59 The Biodegradable Fraction of Natural Organic Matter and its Impact on Bacterial Growth in Potable Water Distribution Systems Raymond M. Hozalski, Apoorva Goel, and Timothy M. LaPara Civil, Environmental, and Geo- Engineering University of Minnesota, Minneapolis, MN, USA Natural organic matter in drinking water supplies is primarily comprised of fulvic acids (FAs; ~80%), humic acids (HAs; ~10%), and bio-polymers like proteins and carbohydrates (~5%), with the remainder consisting of low molecular weight readily biodegradable compounds. The humic substances (i.e., HAs and FAs) are of concern because of their ability to react with water treatment chemicals increasing chemical demand and leading to formation of undesirable disinfection byproducts. The readily biodegradable fraction, however, is also of importance as these compounds can support the growth of heterotrophic bacteria in the water treatment plant and in the water distribution system (WDS). The bacterial biofilms that form on pipe walls and other surfaces in the WDS can become colonized by pathogens and be grazed upon by protozoa. These protozoa, in turn, can serve as hosts for opportunistic pathogens including Legionella pneumophila, known to cause a potentially fatal pneumonia termed Legionnaires’ disease. Controlling bacterial growth in the WDS (i.e., achieving so-called ‘biological stability’) can be achieved through either limiting the availability of the readily biodegradable organic matter, as can be measured by the assimilable organic carbon or AOC assay, and/or by maintaining a robust disinfectant residual (e.g., free chlorine or chloramines). In this investigation, we collected water samples from 18 WDSs across the United States and quantified total organic carbon (TOC), AOC, residual disinfectant, and temperature. The goal was to assess the importance of these metrics as predictors of biological stability in WDSs. Surface water-supplied WDSs exhibited significantly higher (p < 10-4) AOC concentrations (median = 50 µg/L; range = < LOD to 720 µg/L) compared to those supplied by groundwater (median = 16 µg/L; range = < LOD to 156 µg/L). AOC levels were significantly higher (p < 10-4) during the summer months (August 2024 to October 2024) in surface water-supplied WDSs than during the fall (October 2024 to December 2024). In contrast, AOC concentrations in groundwater-supplied WDSs did not vary with season. In surface water-supplied WDSs, AOC concentrations frequently exceeded 50 µg/L, which has been previously suggested as a threshold for biological stability in WDSs containing a residual disinfectant. However, despite these high AOC concentrations in some systems, a stable disinfectant
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